In recent years, for example, in liquid crystal displays and organic EL displays, a method for enhancing the aperture ratio of a display device is known as a method for achieving further higher precision and higher resolution (U.S. Pat. No. 5,953,084). In this method, a transparent planarization film is provided on a TFT substrate as a protective film, to allow data lines and pixel electrodes to overlap on each other, for enhancing the aperture ratio compared with the prior art.
The materials used as the planarization films for TFT substrates are required to have high heat resistance, high transparency and low dielectric constant, and the conventional materials include a material made by combining a phenyl-based resin and a quinonediazide compound (JP7-98502A) and a material made by combining an acrylic resin and a quinonediazide compound (JP10-153854A, JP2001-281853A). However, these materials are insufficient in heat resistance and have a problem that if they are treated at a high temperature as substrates made by using them, the cured films made of them are colored to decline in transparency.
On the other hand, a siloxane polymer is known as a material having high heat resistance, high transparency and low dielectric constant. As materials made by combining a quinonediazide compound with a siloxane polymer to let the siloxane polymer have positive photosensitivity, known are a material made by combining a siloxane polymer having phenolic hydroxyl groups at the ends and a quinonediazide compound (U.S. Patent Publication No. 2003/211407), and a material made by combining a siloxane polymer having phenolic hydroxyl groups, carboxyl groups or the like added by cyclization and heat addition reaction and a quinonediazide compound (JP3-59667A). However, these materials cannot be used as highly transparent materials, for such reasons that since they contain a large amount of a quinonediazide compound or have phenolic hydroxyl groups existing in the siloxane polymer, the coating films formed of them are likely to be opaqued or colored during curing. Furthermore, they also have a problem that the photosensitivity during patterning is low, since they are low in transparency.